The invention is in the field of collapsible frames for stretching sheet material and is specifically directed to a frame of this type which is suitable for home use by hobbyists in fields such as rug-tufting. A particular object of the invention is to provide a frame of this type which is modularly made of repeating, identical pieces that can be inexpensively manufactured by processes such as molding of a thermoplastic material.
In hobby activities, such as making a rug on a burlap base with a rug-tufting needle, it is necessary to stretch the burlap tightly so that it can be easily pierced by the tufting needle to form loops of yarn at the side opposite the side from which it is pierced. Stretching frames to accommodate needs of this type can be and have been made in a variety of ways. For example, burlap has been stretched on canvas frames of the type used for paintings, or on collapsible rectangular frames comprising four sides connected at the corners of the rectangle by screws or nails, or on other similar frames. However, none of the frames of this type known to the inventor herein satisfactorily meet all of the essential requirements for hobby uses, which include easy assembly and disassembly of the frame, compact size for convenient storage when not in use, lightness so the frame can be easily moved from place to place, structural strength so that the sheet material can be stretched tightly, convenient provisions for stretching the sheet material, ease of changing the size of the frame, and the like. This invention is directed to meeting these diverse and sometimes conflicting requirements.
A specific embodiment of the invention comprises a modular, collapsible and adjustable rectangular frame for stretching sheet material, as for rug-tufting by a hobbyist. The frame includes four substantially identical side bars extending along the sides of a horizontal rectangle, at least two substantially identical extension bars extending along at least two opposite sides of the rectangle, and four substantially identical legs extending downwardly from each of the sides of the rectangle. Each bar is integrally formed of a substantially homogeneous material and comprises: an elongated channel member which is substantially rectangular in a vertical section transverse to the length of the bar and is formed of a top and two side walls; a substantially shorter bottom wall at one longitudinal end of the channel member which forms, together with the adjacent portion of the top and side walls a connecting tunnel which is open longitudinally outwardly; a connecting insert extending away from the channel member at the other longitudinal end thereof, said insert being shaped and dimensioned to be received in frictional fit in said connecting tunnel; and a longitudinally extending row of spaced bracing walls each extending transversely of the top and side walls and being integral therewith. A selected pair of adjacent bracing walls form, together with the portions of the top and side walls intermediate thereto, a leg tunnel which is substantially rectangular in horizontal section. The side wall facing away from the frame has at its lower portion a longitudinally extending row of web-engaging projections facing away therefrom. The connecting insert of each side bar extends transversely of the length of the channel member thereof and the connecting insert of each extension bar extends along the length of the channel member thereof. Each leg is integrally formed of a substantially homogeneous material and comprises an upper tubular portion which is substantially rectangular in horizontal section and is dimensioned and shaped to be slidably received in frictional fit from below into a leg tunnel, and a tapered lower portion. The frame is assembled by frictionally fitting the connection insert of each bar into the connecting tunnel of another bar to form a rectangular frame, and by frictionally fitting each leg into a leg tunnel of each side of the rectangular frame. Sheet material is stretched by laying the material over the frame and pulling outwardly the marginal portion thereof by a web laced through the material and lacing said web-engaging projections. The length of any pair of opposite sides of the frame is changed by adding or subtracting extension bars.